1. Field of the Invention
The present invention relates to devices for grinding raw materials and can be used in power engineering, in construction, mining, chemical and other industries. The invention can be most advantageously employed in crushing devices designed to prepare the solid organic fuel for burning and gasification in fluidized-bed thermal power plants.
2. Description of the Related Art
The effective operation of fluidized-bed plants implies the use of a fuel with a relatively narrow fraction composition.
To combat unburned carbon and carryover loss, it is particularly important that the fuel used in such plants contain the lowest possible amount of fine fractions (smaller than 1 mm).
So the crushing devices designed to prepare the fuel for burning and gasification, preferably in fluidized-bed thermal power plants, must provide as uniform crushed product as possible, with a minimum fine-fraction content.
Known in the art is a crushing device comprising a casing accommodating a rotor having a vertical axis of rotation and formed by a disk with fractionating blades and movable grinding elements. The latter are beaters rotatably mounted about the vertical axes and extending radially beyond the rotor disk. In addition, stationary grinding elements in the form of baffle plates are mounted on the casing of the device coaxially with the rotor (U.S. Pat. No. 3,860,184).
In this apparatus, the material is crushed in the following way. The raw material is fed, through an inlet duct, onto the central part of the rotating disk of the rotor, is acted upon by Coriolis force to thrust it against the working faces of the fractionating blades, is then transported therealong to the periphery of the rotor disk by centrifugal forces and flung outwardly to the reflecting surface of the stationary grinding elements. The material is ground as a result of striking against the stationary grinding elements. The material subjected to primary crushing is brought down along the walls of the crusher casing, strikes the beaters of the movable grinding elements, and is further ground by abrasive action within the space between these elements and the casing.
The final crushed product drops down and is discharged from the plant through an outlet duct. In this device, the maximum size of the pieces in the crushed product is generally controlled by the rotational speed of the rotor, i.e. by the impacting forces between the pieces of the material and the stationary grinding elements.
Metallic and other non-crushable bodies that have got into the crushing device are flung off, together with the material, under the action of centrifugal force, towards the inner wall of the casing. The non-crushable bodies, on striking the rotatable beaters, deflect them, fall down from the grinding zone, and are discharged along with the final crushed product. The deflected beaters are returned by centrifugal force to the initial position.
So in this crushing device, by virtue of making each beater rotatable about the vertical axis, the beaters are protected from damages as a result of impacting non-crushable objects, and in addition, these objects are removed from the grinding zone, thus ensuring a relatively high reliability both of the grinding elements and of the entire rotor of the crusher.
Since the grinding of the material, however, is here accomplished due to impacting and abrasive action and the crushing size is largely controlled by the rotational speed of the rotor, the final product will contain an increased amount both of coarser fractions (due to different strengths of the pieces) and of finer fractions (due to abrasion), which leads to a non-uniform crushed product with a relatively high fine-fraction (less than 1 mm) content of the product.
Besides, extra unproductive power expenses are required for overgrinding.
Furthermore, the non-crushable bodies that have found their way into the crushing device, mix with the final crushed product, when the rotatable beaters are deflected, resulting in a poorer quality of the product.
In addition, during operation of the crusher, the working surface of the rotor disk, the fractionating blades, and the movable and stationary grinding elements are worn out, leading to a shorter life of the crushing device and making it less reliable.
It will be noted, that the end faces of the beaters (i.e. the movable grinding elements) particularly suffer from wear. Consequently, in operation, the beaters often have to be replaced, so that their life is relatively short, resulting in extra metal expenses.
Known in the art is a crushing device comprising a casing accommodating a rotor having a vertical axis of rotation and formed by a disk with radial fractionating blades placed on the working surface thereof, including each a working lateral face and consisting of a main portion rigidly secured to the disk and located nearer to the rotor axis, and a peripheral portion attached to the rotor disk by a hinge and rotatable about the vertical axis, and stationary grinding elements mounted on the casing coaxially with the rotor disk and formed by projections facing the rotor axis. The projections are disposed above the disk, and more specifically, above the peripheral portions of the fractionating blades, and spaced from the working surface of the disk by a distance "H" corresponding to the predetermined maximum size of the pieces in the final crushed product. The device further includes a rotor drive (SU, A, 1,217,467).
The distance between the working surface of the rotor disk and the projections is dependent on a number of factors, such as the kind of material, the operating conditions of the crusher, etc. Therefore, it is determined experimentally, depending on the required maximum size of the pieces in the crushed product.
The crushing device operates as follows. The raw material is fed through the inlet duct onto the central part of the rotating disk of the rotor, is thrust by Coriolis force against the working faces of the radially extending fractionating blades, and is then transferred, under the action of centrifugal force, along the blades towards the periphery of the rotor disk. The working faces of the fractionating blades are those directed in the rotating sense of the rotor.
When the material has reached the periphery of the rotor disk underlying the projections, the pieces of a size exceeding the space between the working surface of the disk and the projections are subjected to grinding. The lower part of the pieces is then thrust against the blades, while the upper part thereof, on impacting the projections, is chipped off by the lower edges of the blades at a height determined by the specified maximum size of the pieces in the crushed product.
The lower, chipped-off, part of the pieces is flung off towards the inner wall of the device, drops along said wall, and is discharged from the device through the outlet duct, whereas the remaining upper part of the pieces, gradually sinking, is subjected to further grinding. The pieces of a size less than the required maximum size of the pieces in the final crushed product pass under the lower edges of the projections without being crushed, and are flung off towards the casing wall, dropping the realong to be also discharged through the outlet duct.
So in such crushing device, in contrast to the device disclosed in U.S. Pat. No. 3,860,184, the grinding of the material is largely performed through the mechanism of shearing, the maximum size of the pieces in the product being controlled by the height at which the material is sheared, which substantially reduces the amount of both the coarser and the finer fractions in the final crushed product, thus resulting in a more uniform end product.
In this case, since the material crushing is accomplished essentially by shearing, the rotational speed of the rotor is set at a much lower value (by a factor of two to three) than that required for impact crushing, in the device of U.S. Pat. No. 3,860,184, and so the pieces that have passed between the lower edges of the projections and the working surface of the disk without being crushed, practically, are not overground as a result of striking the casing wall. On the other hand, the larger lumps subjected to crushing by shear force are slowed down in the crushing zone and arrive at the wall of the crusher casing at a low speed which is insufficient for impact destruction, again leading to a relatively low amount of fine fractions in the final product.
The non-crushable objects, however, that happen to get into the crushing device and whose size exceeds the spacing between the working surface of the rotor disk and the projections, as they impact the rotary peripheral portions of the blades and cause them to be deflected, fail to pass under the projections and, staying on the rotor disk, they repeatedly strike both the projections and the fractionating blades, entailing the risk of their damage and consequently reducing operational reliability of the rotor and the entire device.
Moreover, in use, the non-crushable bodies are accumulated on the rotor disk, leading to abrasion of the material between these bodies with the consequent increase of the amount of fine fractions (i.e. less than 1 mm in size).
Further, since in this crusher, the stationary grinding elements in the form of projections are located above the peripheral portion of the disk plane, the material is liable to abrade between the lower surfaces of the stationary projections and the working surfaces of the rotary disk, again resulting in a greater amount of fine fractions in the end product. An increased amount of finer fractions in the end product, due to abrasion, is especially noticeable when crushing rather brittle materials.
In addition, to provide an effective crushing of the material, i.e. to ensure high local stresses as it is being crushed by shearing, sufficiently high crushing forces must be exerted. The higher crushing forces can be obtained by raising kinetic energy, and hence the mass of the movable grinding elements, for example, by increasing their thickness, while maintaining their length and width within structurally allowable limits.
In this crushing device, however, the maximum thickness of the rotary peripheral portions of the fractionating blades, which are located on the rotor disk and serve as movable grinding elements, is limited by the distance between the working surface of the disk and the projections, which is determined by the required maximum size of the pieces in the crushed product. In fact, the mass of the movable grinding elements may prove insufficient and they will tend to deflect during the crushing process, resulting in an additional abrasion of the material on the disk and in a relatively low efficiency of destroying the material by shearing.
Besides, the overgrinding of the material, similarly to the previous design, leads to excessive power consumption.
Furthermore, here, just as in the above mentioned design, an intense wear occurs, in operation, both of the rotor itself, i.e. the surface of the disk and the fractionating blades, and of the stationary grinding elements, shortening the life of the crusher and impairing its operational reliability.
Among the members most sensitive to wear are the grinding elements, i.e. the peripheral portions of the fractionating blades and the projections, these elements undergoing non-uniform wear in operation. Such non-uniform wear of the grinding elements results from the fact that the edges of said elements are primarily affected by the material being crushed, i.e. the lower lateral edges of the projections and the adjacent upper lateral edges of the peripheral blade portions. The wear of said edges of the grinding elements causes their geometry to be changed, bringing about an increased number of fractions larger in size than specified in the end product.
To avoid non-uniformity of the final crushed product and to extend the life of the grinding elements, in some crushers they are turned through 180.degree..
In the aforementioned device, it is possible to turn the projections through 180.degree., but only two opposite edges of the projections will be used in operation, which fails to prolong their operational life to a full extent.
The location of the peripheral blade portions, which are the movable grinding elements, on the rotor disk makes it difficult to turn them through 180.degree., since in this case, clearances will be formed between the worn edges of the peripheral blade portions and the working surface of the disk, and the material may get into these gaps, resulting in its undue abrasion and an additional abrasion of the peripheral portions of the blades. Therefore, in such device, like the previous design of U.S. Pat. No. 3,860,184, during operation of the crusher, the grinding elements, particularly the peripheral portions of the blades, have to be rather frequently replaced, which reduces their life and results in extra costs for metal.